1. Technical Field
This invention relates to a semiconductor element having an ohmic contact construction and a method of producing that element.
2. Background of the Prior Art
As an example of a semiconductor element, the following discussion will focus on a luminous element emitting light from its upper surface, having a very short path of emission, and composed of materials of the AlGaAs family.
In the AlGaAs family, the ohmic contact resistance can be easily reduced by using GaAs without the Al, and an n-type material. Accordingly, an n-type GaAs material is widely used as the cap layer to achieve electrode and ohmic contact.
FIGS. 7A through 7E show a process used to produce a luminous element emitting light from its upper surface. More particularly, FIGS. 7D and 7E show, respectively, a cross section of the completed luminous element and an overhead view of its upper surface.
As shown in FIG. 7A, a p-type AlGaAs lower cladding layer 41, a GaAs active layer 42, an n-type AlGaAs upper cladding layer 43 and an n-type GaAs cap layer 44 are successively grown on a p-type GaAs substrate 40. A circular pattern is then created in FIG. 7B in the center portion of the upper surface of cap layer 44 for the luminescent area. The pattern is formed by photolithography using an AZ-resist 45 as a mask. The unmasked area is then removed by etching down to the depth of the lower cladding layer 41. After etching, the entire surface is covered with an insulating film consisting primarily of SiO.sub.2 46.
As shown in FIG. 7C, the AZ resist 45 and the insulating film above it are then removed. Next another circular pattern, smaller than the aforementioned AZ resist 45, is formed on cap layer 44, again using AZ resist as a mask (not pictured). This AZ resist allows an electrode 47 for the n-side to be fused by vapor deposition to the periphery of cap layer 44 and the entire surface of insulating film 46. The aforementioned AZ resist is then removed. By this process, light emission window 47a is formed, and the n-side electrode 47 is brought into ohmic contact with cap layer 44 in an annulus 47b around window 47a.
Because the n-type GaAs cap layer 44 functions as an absorption layer for the emitted light, the central portion of cap layer 44 is removed as seen in FIG. 7D by etching, leaving only the peripheral portion lying beneath the ohmic contact annulus 47b. Thus, as shown in top view in FIG. 7E and by cross-section in FIG. 7D, the light emission window 44a is extended through cap layer 44.
In order to shorten the path of emission in this type of element, which emits light from its upper surface, the area surrounding the portion emitting light is etched away with the result that the current flow is confined to a constricted region. Correspondingly, the ohmic contact with the electrode is restricted to the portion 47b of the uppermost layer.
For the crystal in the uppermost layer, a material must thereby be chosen which can minimize the contact resistance, and the surface of the element must be covered with this material. However, since this material will absorb light, it will reduce the output of the light emitting element. Thus, it is necessary to use a process (i.e., the creation of window 44a) which will remove, by etching or some similar procedure, the portion of the surface which overlies the area of emission.
If the path of emission is made shorter, the area of the ohmic resistance portion will also be reduced. As a result, the driving voltage will increase, and because of increased heat dissipation, an anticipated improvement in light output will not occur.
In order to judge the quality of ohmic contact, the contact resistance (.OMEGA..multidot.cm.sup.2) is evaluated as to whether it is larger or smaller than 10.sup.-5 ; the smaller the contact resistance, the better the contact. With the aforementioned luminous element which emits light from its upper surface, Rc=5.times.10.sup.-6 (.OMEGA..multidot.cm.sup.2), the inner diameter of ohmic contact portion 47b is 80 .mu.m. and its outer diameter is 100 .mu.m. Thus, the area of contact is approximately 3.times.10.sup.-5 cm.sup.2, and the contact resistance is approximately 0.17 .OMEGA..
If the area of light emission is made even smaller, the ohmic contact area will be further reduced, resulting in an additional further increase in contact resistance. Any increase in the resistance component is expressed as a greater production of heat, which lowers the output of the luminous element.
Several examples of luminous elements exist which emit light from their edges. These elements use a diffusion process to form the constricted region in which current can flow.
FIG. 8 shows an example of a Channeled Substrate Planar laser ("CSP laser") design. A CSP laser is constructed by first forming a groove on an n-type GaAs substrate 50 by photolithography or another similar process. To this substrate are then added, successively: an n-type AlxGa.sub.1-x As lower cladding layer 51; a p-type AlyGa.sub.1-y As active layer 52; a p-type Al.sub.x+ Ga.sub.1-x As upper cladding layer 53; and an n-type GaAs cap layer 54. The p+-type Zn diffusion area 55 is formed in cap layer 54 and upper cladding layer 53; the p-side electrode 57 is formed on the growth surface, and an n-side electrode 56 is formed on the substrate's undersurface.
In this way, a portion of the uppermost n-type layer, such as cap layer 54, and a portion of the p-type layer are interchanged through the use of a Zn diffusion process, thus forming a restricted region in which current can flow. Further, the entire surface of cap layer 54 is covered by a p-type electrode 57, which makes ohmic contact with the narrow area 55 where the p-layer and n-layer are interchanged.
As stated previously, the quality of ohmic contact is judged by the Rc. For example, if Rc=5.times.10.sup.-6 (.OMEGA..multidot.cm.sup.2), the area of region 55 where the p-layer replaces the n-layer will be 1.5.times.10.sup.-5 cm.sup.2, and the contact resistance will be approximately 0.33 .OMEGA.. If the area where the p-layer replaces the n-layer is made even smaller, in order to reduce the luminous area, the resistive component resulting from the contact resistance will increase.
FIG. 9 is a graph illustrating how the forward voltage and current characteristics depend on the Rc in a luminous element. As the contact resistance increases, it causes the driving voltage to increase. The increase in contact resistance becomes evident as an increase in the calorific value which causes a decrease in the output of the luminous element.
When a diffusion process is used, the conductivity types are interchanged on a portion of the uppermost layer. This construction yields a luminous element with a constricted current path which emits light from its edges. Some other examples of this type of luminous element are Terraced Substrate lasers ("TS lasers") and Transverse Junction Stripe lasers ("TJS lasers"). However, these elements, too, are constructed by directly covering an area of reversed polarity with an ohmic electrode. This gives rise to problems in minimizing the contact resistance. If the area of reversed polarity is reduced, the increase in contact resistance will cause the driving voltage to increase, and the increased generation of heat will prevent any improvement of luminous output.